1. Field of the Invention
The present invention relates to a diamond single crystal substrate manufacturing method and a diamond single crystal substrate, especially a diamond single crystal substrate manufacturing method and a diamond single crystal substrate, having a high quality, large size and no unintentional impurity inclusions, and suitable for use as semiconductor materials, electronic components, optical components, or the like.
2. Description of the Prior Art
The many outstanding properties of diamond, such as high thermal conductivity, high electron/hole mobility, high dielectric breakdown field, low dielectric loss and wide bandgap, make it an unparalleled semiconductor material. In recent years, in particular, ultraviolet light emitting devices that take advantage of diamond's wide bandgap, and field effect transistors having excellent high frequency characteristics, etc. have been developed. Thanks to its transparency extending from the ultraviolet to the infrared region, diamond shows promise as well as a material for optical components.
In order to use diamond as a semiconductor, it is necessary to make it into high-quality, large-size single crystal substrates, as with other semiconductor materials. Diamond single crystals are obtained at present chiefly by high-temperature high-pressure synthesis methods, which result in superior crystallinity compared with natural single crystals, but that suffer from the inclusion of nitrogen impurities in the crystals unless special growth conditions are used. A substrate containing nitrogen is difficult to use on its own as a semiconductor single crystal, for which reason seed substrates are often used for growing diamond single crystals using a vapor-phase synthesis method that reduces impurity inclusions (see for instance, Japanese Patent Publication No. 2003-277183A).
Also, the size of the diamond single crystals obtained by the high-temperature high-pressure synthesis methods is limited to a grade of 1 cm, and it is difficult to achieve large-size single crystals with diameters of not smaller than 10 mm. Research is being conducted therefore in order to manufacture a larger surface area single crystals by vapor-phase synthesis, wherein the single crystals above are used as seed substrates. For instance, Japanese Patent Publication No. 03-75298A discloses a method for manufacturing diamond single crystals by arranging the orientations of a plurality of single crystal diamonds in approximately the same direction and then growing a diamond thereon by vapor-phase synthesis thereon. For obtaining large-size diamond single crystals using such a method, Japanese Patent Publication No. 07-17794A discloses that, by controlling the crystal orientation, spacing and height of a plurality of single crystal diamonds and restricting the growth temperature to an adequate range in order for homoepitaxial growth to be maintained up to a predetermined thickness, a high-purity vapor phase synthesis is achieved that allows providing large-size diamond single crystals for optical or semiconductor use, with 15 mm or larger diameters and having a good crystallinity, supported by ultraviolet transparency near the 250 nm wavelength region, X-ray rocking curves with a half-width value of 100 seconds or less, Raman scattering spectra with a half width of 2 cm−1, etc.
As described in Japanese Patent Publication No. 2003-277183A, when diamond single crystals are grown by vapor-phase synthesis on a diamond single crystal substrate obtained by a high-pressure synthesis process, residual stresses accumulate in the vapor-phase growth layer. Residual stresses remaining in the vapor-phase growth layer are problematic in that the associated crystal strain may alter semiconductor properties such as bandgap, mobility, etc. These phenomena are not solved by the vapor-phase growth of diamond single crystals from seed substrates 100 μm or less thick, as disclosed in Japanese Patent Publication No. 2003-277183A. Also, diamond single crystal substrates obtained by formation of thick films having a thickness of 100 μm or more by vapor-phase synthesis may threaten with substrate cracking due to accumulated stresses. Owing to the increased probability of substrate cracking entailed by substrates having a larger size (larger surface area, thicker film), the above problems in obtaining large-size diamond single crystal substrates, as described in particular in Japanese Patent Publication Nos. 03-75298A and 7-17794A, are not essentially solved using a method as described in Japanese Patent Publication No. 03-75298A, wherein large-size single crystal substrates are obtained by integrating in a single unit substrates consisting of an arrangement of a plurality of high-pressure phase substances having essentially mutually identical crystal orientations and acting as vapor-phase growth nuclei upon which is then grown a single crystal by vapor phase synthesis, or using a method as described in Japanese Patent Publication No. 7-17794A, wherein large-size single crystal substrates are obtained by controlling the single crystal growth conditions.
Also, the practical implementation of large-size diamond homoepitaxial growth by conventional techniques such as those described in Japanese Patent Publication Nos. 03-75298A and 7-17794A is plagued by problems of unintentional impurities becoming enclosed in the crystal. When unintentional impurities become trapped in the crystal, an impurity level is reached which is incompatible with the targeted use as semiconductor, etc. and that impairs not only its use as a semiconductor substrate, but restricts as well, due to changes elicited in the optical properties of the crystal, its use as an optical substrate. Elements enclosed as impurities in diamond single crystals grown homoepitaxially by chemical vapor-phase synthesis include mainly hydrogen, silicon, nitrogen, boron, etc.; among these, nitrogen impurities introduced unintentionally in the crystal are the most influential as regards semiconductor and optical properties. Nitrogen is the element most difficult to control as an impurity, since it is an essential component of the atmosphere and is therefore present in non-negligible amounts inside vacuum vessels. Thus in order to manufacture diamond single crystal substrates for semiconductor or optical use by vapor-phase synthesis it is vital to assess and control nitrogen impurities; however, at present it is extremely difficult to predict semiconductor and optical properties by regulating the amount of impurity inclusions through a strict control of the factors that affect their uptake. Thus far, methods for obtaining diamond single crystals by heteroepitaxial methods result in large numbers of crystal defects and therefore in an insufficient quality for warranting use as optical or semiconductor substrates.